Terahertz MEMS metadevices

One of the initial demonstration of THz MEMS metadevice was reported using thermally reconfigurable bimaterial cantilevers. The metamaterial unit cell comprised of gold (Au) split ring resonator (SRR) on silicon nitride (SiN) membrane as shown in figure 1(a) [109]. The silicon (Si) substrate underneath the unit cell was completely etched away, leaving the SiN membrane suspended with two cantilever anchors, attached to the substrate. The cantilevers were made of bimaterial stack of Au/SiN layers. Upon fabrication, the released SiN membranes housing the Au SRRs remained relatively flat as shown in figure 1(b). Rapid thermal annealing was used to anneal the fabricated metadevice from 350 °C to 550 °C, in steps of 50 °C. After each annealing step, the bimaterial cantilevers were permanently curved up with a larger deformation as shown in figure 1(c). The released membranes housing the SRRs were lifted from ∼0° (flat) after fabrication to ∼65° (almost upright position) after annealing at 550 °C. This resulted in a large change in the THz response of the metadevice after each annealing step for both E_X and E_Y incidence of THz waves as shown in figures 1(d) and (e), respectively. Hence, by incorporating thermally driven bimaterial cantilevers with SRR unit cells, a large change in anisotropic THz response was demonstrated through reorientation of metadevice. This approach was also adopted to demonstrate spatially selective reconfiguration of unit cells by placing the bimaterial cantilevers on desired unit cells, while the other unit cells were placed on an anchorless SiN membrane that were non-deformable [110]. Upon annealing, only the unit cells suspended with bimaterial cantilevers were deformed and allowed for spatially selective reconfiguration of the THz MEMS metadevice. Hence, large and continuous deformation of unit cell geometry with spatially-selective control was demonstrated using thermal annealing of bimaterial cantilever approach. However, this mechanical reconfiguration is caused due to the changes in residual stress in the bimaterial layers forming the cantilevers and so, the deformation of the cantilever after each annealing cycle is non-volatile and cannot be reversed. This limited the operation of these metadevices to a single cycle. The proposed approach also suffers from high power consumption and requires furnace to anneal the sample at such high temperatures.

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Multicycle operation of thermally reconfigurable THz MEMS meta device was also reported using bimaterial cantilevers comprising of materials with largely different coefficient of thermal expansion [111]. Temperature driven differential expansion/contraction of the constituent material layers, results in an overall out-of-plane deformation of the bimaterial cantilever. The direction of out-of-plane deformation (i.e. upwards or downwards) with increasing temperature will be determined by the relative placement of the two materials forming the cantilever. Bimaterial cantilevers made of aluminium (Al) and aluminium oxide (Al₂O₃) stack were formed as a part of electrical SRRs (ESRR) on top of Si substrate as shown in figure 1(f). Upon fabrication, the bimaterial cantilevers were deformed upwards (away from the substrate), owing to the residual stress in the bimaterial layers as shown in figure 1(g). The measured resonance frequency of the ESRR at room temperature (300 K) was at ∼0.4 THz as shown in figure 1(i). When the temperature of the metadevice was gradually decreased from room temperature to 77 K, the bimaterial cantilevers curved upwards as shown in figure 1(h). This led to a continuous blue-shift of the resonance frequency until 0.42 THz at 77 K, as shown in figure 1(i). Alternatively, when the temperature was gradually increased to 400 K, the cantilevers continuously curved downwards as shown in figure 1(h), leading to a red-shift in resonance frequency upto 0.32 THz as shown in figure 1(i). The THz resonance spectral tunability of ∼0.1 THz was achieved for the overall temperature change of 323 K. Hence, a continuous, bidirectional and repeatable operation of thermally driven THz MEMS metadevice was experimentally demonstrated. Even though, multicycle operation of metadevice was demonstrated using thermal actuation, the need for external cooling and heating systems, higher power consumption and slower speed are some of the major technological challenges that needs to be addressed.

Electrical current driven Joule heating of resistive elements can enable highly compact solutions, while providing all the functional benefits of thermally reconfigurable bimaterial cantilevers. Electrically connected 2D array for bimaterial cantilevers was reported for electrothermal reconfiguration of THz resonance frequency as shown in figure 2(a) [112]. The bimaterial cantilever was made of Al and Al₂O₃ material stack and was deformed upwards upon fabrication due to residual stress. The measured resonance frequency of the as-fabricated metadevice without input current (0 mA) was at 0.85 THz as shown in figure 2(b). When electrical current was passed through the Al lines, the temperature of the entire metadevice increased due to Joule heating. This rise in temperature led to the downward deformation of cantilevers, thereby causing the resonance frequency of the metadevice to continuously red shift until 0.6 THz for 550 mA of input current as shown in figure 2(b). Similar approach was earlier adopted in a released 'Ω'-ring enclosing a disk resonator based metadevice as shown in figure 2(c) [113].

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Electrothermal actuation was also reported for active control of THz absorption [114]. Metamaterial absorbers are made of three layers—bottom reflector, dielectric spacer and top metamaterial layer. The metamaterial unit cell consisted of Au SRR on SiN plate, which was suspended by two bimaterial cantilevers made of Au/SiN with an air gap, over Au coated Si substrate which acted as the reflector, as shown in figure 2(d). The air gap between the bottom Au and top metamaterial layer formed the spacer layer. Tungsten heating resistors with thermal isolations were integrated within the unit cell to enable efficient heating of the bimaterial cantilevers. The temperature of the device was increased by passing current through the tungsten heater. With increasing temperature, the bimaterial cantilevers bent downwards, thereby reducing the air gap between the top SRR layer and bottom Au reflector. This consequently led to a reduction in absorption at ∼0.68 THz by 30% for the input current of 1200 mA as shown in figure 2(e). Hence, electrothermal approach is highly attractive for integrated solutions with large and continuous deformation. However, it still suffers from high power consumption, slower speed and high sensitivity to temperature and pressure variations.

Complementary to the out-of-plane reconfiguration, thermal actuators can also enable in-plane reconfiguration i.e. in the plane of the metamaterial. In-plane reconfigurable thermal actuators are made from single material layer. The popular choices of thermal unimorph actuator designs are the V-beam and U-beam actuators. The V-beam actuator also known as 'chevron actuator', is made of a long beam that is fixed on either side with a slight bend at the mid section, hence forming a V-shape [115]. When electrical current is passed through the V-beam, the Joule heating increases the temperature of the V-beam. This leads to the expansion of the V-beam and the mid section of the V-beam is pushed forward. Adoption of such in-plane reconfiguration for THz MEMS metadevice has been theoretically proposed [116]. The design proposed the use of metallic inclusion attached to the mid-portion of V-beam actuator integrated with SRR unit cells. By moving the metallic inclusion on the V-beam actuator, the effective capacitance of the SRR gap could be altered, thereby leading to a corresponding spectral tuning of SRR resonance frequency. Another popular unimorph thermal actuator for in-plane reconfiguration is the U-beam actuator and was also proposed for THz MEMS metadevice [117]. U-beam actuator consists of two geometrically dissimilar beams connected in the shape of 'U'. Upon passing electrical current, the two beams forming the U-beam heats up by varied proportion, dictated by their respective geometrical resistance. This leads to differential expansion between the two beams. Since, the two beams are mechanically connected, an overall in-plane deformation of the U-beam is achieved. A variation of U-beam called the two-hot arm actuator was proposed for resonance frequency tuning of THz MEMS metadevice [117]. The two hot-arm actuator consists of two thin arms connected to third thicker arm. This design variation provided larger deformation for a given current stimulus. Two of these actuators with thick arm made of metal is placed with a small gap from a SRR unit cell gap. Upon applying electrical current, the metallic part of the two-hot arm actuators move toward the SRR gap and reduces the effective capacitance. This results in a continuous tuning of SRR resonance frequency. Even though, the theoretical proposals highlight the potential of in-plane reconfigurable thermal actuators for THz MEMS metadevices, there are some important fabrication-related challenges that need to be addressed for the realization of these metadevices, such as high sensitivity to stress in longer beams, inherent residual stress and temperature dependent out-of-plane deformation in bilayer beams.

The simplest of the electrostatic actuator is a conductive cantilever fabricated on top of a lightly doped silicon substrate with an insulating layer in between. An array of cantilevers made of Al/Al₂O₃ bimaterial stack, fabricated on top of lightly doped Si substrate was reported to provide excellent dynamic THz performance [118]. The cantilevers were prestressed due to the residual stress between the bimaterial layers and were bent upwards after the release process, as shown in figure 3(a). This allowed for a much larger tip displacement of the released cantilevers (in µm), compared to the sacrificial silicon oxide (SiO₂) layer thickness of only 100 nm. It was also shown that by decreasing the thickness of Al layer, while keeping the Al₂O₃ thickness unchanged, resulted in an enhancement of initial tip displacement of the fabricated cantilevers. This consequently increased the tunable range of the resonance frequency of the cantilever-based THz MEMS metadevice. The dipole resonance of the cantilever metadevices for Al thickness of 100 nm, 300 nm and 500 nm were measured to be 0.88 THz, 0.935 THz and 0.96 THz, respectively as shown in figure 3(b). By applying voltage between the Al layer and Si substrate, the cantilevers were deformed towards the substrate, until it came in complete physical contact with the substrate. In this configuration, the ON state resonance for all three metadevices was ∼0.6 THz. Maximum switching contrast of 0.6 was achieved at ∼0.6 THz for all three metadevices as shown in figure 3(c). Hence, a simple cantilever array fabricated on lightly doped Si substrate acted as an efficient THz switch. The geometrical parameters of the cantilevers and the thickness of constituent materials could be engineered to achieve desired tunable THz response. Electrostatic actuated cantilever array was also reported for continuous spectral tunability of resonance frequency, by gradually varying the gap between the substrate and the released cantilevers [119]. In another report, simultaneous tuning of multiband resonance was demonstrated using triple cantilever resonator as a unit cell of the THz MEMS metadevice [120].

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The simplicity of cantilever design and integration allows for selectively releasing a part of any resonator design and then by electrically connecting them will enable active structural reconfiguration using electrostatic actuation. Hence, the cantilever approach was reported for a wide range of reconfigurable THz functionalities. In an initial work, the side arms along with the tip arms of dual SRR unit cell were released to form 'L'-shaped cantilevers as shown in figure 3(d), to enable electrostatically tunable magnetic response at THz frequencies [121]. Alternatively, active control of resonant electrical response was reported by releasing the 'T'-shaped cantilever formed from the centre arms along with the capacitive tip arm of the ESRR unit cell as shown in figure 3(e) [122, 123]. Furthermore, polarization independent response was also reported for all reconfiguration states using a unique metamaterial unit cell design that consisted of eight cantilevers placed at each corner of electrically connected octagon ring, as shown in figure 3(f). The metadevice response for both E_X and E_Y polarized THz incidence were identical at all reconfiguration states [124]. This was achieved due to the high symmetry of octagon-shaped unit cell design as well as the out-of-plane reconfiguration of cantilevers, which preserves the symmetry of unit cell in all states. Electrostatic cantilever integrated metadevice was also reported for dynamic polarization control of THz waves [125]. The unit cell design consisted of cantilever array that can be reconfigured to come in contact with an orthogonally oriented metal line as shown in figure 3(g). The metadevice response for THz incidence polarized along the cantilever length was varied strongly, while the metadevice response for orthogonal polarization remained relatively unchanged. This anisotropic response was then utilized to dynamically alter the polarization of THz waves between circular and linear states. Furthermore, electrostatic cantilever was also integrated in a metal–insulator–metal (MIM) cavity configuration as shown in figure 3(h) for dynamic phase control [126]. The array of cantilevers formed the top metamaterial layer, while the bottom reflector was formed by a continuous Al film on Si substrate. The spacer layer was a stack of thick SiO₂ layer, 50 nm thin Al₂O₃ layer and a tunable air gap. Hence, by applying the voltage between the bottom Al and top cantilever, the air gap and consequently the effective spacer thickness was dynamically varied. This allowed for the switching of metadevice between over-damped and under-damped regions with corresponding phase span of full 360° and less than 180°, respectively. Such level of dynamic phase control was reported to enable THz polarization control, beam steering and holographic beam forming applications through simulation studies [127]. The MIM architecture with metmaterial as top layer was also reported for tunable THz absorption [128]. The metadevice unit cell consisted of 'I'-shaped resonator fabricated on SiN membrane that was suspended with two cantilevers that were anchored to a fixed rectangular ring. The air gap between the top metamaterial layer and bottom reflector was reduced by applying voltage between them and this led to the gradual red-shifting of peak absorption frequency of the THz MEMS metadevice.

The fabrication process used to realize electrostatic cantilever readily allows for two isolated electrical control of the metadevice [129]. This opened a new avenue for advanced manipulation of THz waves as shown in figures 3(i)–(m). In the initial report, electrically isolated SRR and ESRR unit cells placed in an interpixelated fashion as shown in figure 3(i), was reported for independent control of magnetic and electrical resonances [130]. Then, independent reconfiguration of bright and dark mode resonators in near-field coupled metadevice was reported for amplitude modulation and spectral tuning of electromagnetically induced transparency (EIT) analogue as shown in figure 3(j) [131]. Similar approach was also employed for the realization of Fano resonance through symmetry breaking in the wave propagation direction [132]. Two SRRs with independent control formed the Fano resonator unit cell as shown in figure 3(k). By changing the height of the cantilevers in each SRR independently, the strength of the Fano resonance was modulated. Interestingly, the presence of Si substrate broke the mirror symmetry of the metadevice in the vertical direction and enabled metahysteresis behaviour. This unique feature was used for demonstrating various logic gate operations at THz frequencies [132, 133]. In another work, complete control of anisotropic response was reported by isolating the control to orthogonally twisted cantilevers placed in a ring resonator configuration as shown in figure 3(l) [134]. Based on the combination of cantilever switched and polarization of incident THz waves, varied linear polarization response was achieved at specific THz frequency. Active switching of chirality between achiral, levorotatory, dextrorotatory and racemic states was also reported by using isolated control of orthogonally twisted 'L'-shaped cantilevers as metadevice super cell as shown in figure 3(m) [135]. Various other functionalities were also reported using the electrically isolated electrostatic cantilever platform including switching of EIT analogue in tri-atomic metadevice [136], THz bandwidth control [137], tunable filter [138], and tunable dual band absorption [139]. The two lithography step fabrication process limits the control to only two isolated electrodes for a metadevice. However, the number of metallic layers can be scaled up and this will enable even higher level of isolation for extreme THz wave manipulation.

Even though the microcantilever on doped Si substrate enables simpler implementation of electrostatically driven THz metadevices, it suffers from additional loss due to substrate doping, limited reconfiguration range, and reduced reliability.Alternative electrostatic schemes for out-of-plane reconfiguration are also explored to address the aforementioned challenges. Electrostatically actuated microplate array, fabricated on quartz substrate was reported to perform as an efficient THz filter and modulator [140, 141]. The metadevice unit cell consisted of two SiN coated Au plates—one fixed on the quartz substrate, while the other was suspended above the fixed plate with an air gap between them (OFF-state), as shown in figure 4(a). When voltage was applied between the two Au plates, the electrostatic force pulls the suspended plate towards the fixed plate. Beyond the critical pull-in voltage, the gap between the two plates was completely closed (ON-state), as shown in figure 4(b). Thin coating of SiN dielectric on both Au plates prevents the electrical short between the two Au plates upon physical contact. The metadevice showed a large switching contrast of over 70% at 0.48 THz for an applied voltage of ∼33 V, as shown in figure 4(c). The large surface area of the plates enabled lower actuation voltage, even with a large air gap between the two plates of ∼5.8 µm. Such large gaps are required to achieve high enough restoring force that would overcome the adhesion forces of the SiN coated Au plates, and bring the cantilevers back to their initial OFF state. This is important for ensuring repeatable operation of the electrostatically driven THz MEMS metadevices. Also, the use of quartz substrate alleviated the substrate related loss incurred in doped Si substrate based metadevices. In another report, Al/SiN bimaterial cantilevers were fabricated on top of the Au ESRR on semi insulating gallium arsenide (GaAs) substrate as shown in figures 4(d) and (e). Electrostatic actuation was used to electrically short the ESRR gap by pulling down the cantilever [93, 142]. This allowed for the dynamic switching of LC resonance at 0.75 THz with a contrast of over 50% as shown in figure 4(f). Integration of THz MEMS on GaAs substrate could also be used for reconfigurable THz sources and detectors. Complementary to fixed-free cantilevers, electrostatically driven fixed-fixed beams were also reported for the realization of THz MEMS metadevices [143, 144]. In this report, vertically aligned Au beam was suspended over horizontally aligned Au lines that were fabricated on SiO₂ coated Si substrate as shown in figures 4(g) and (h). When voltage was applied between the suspended Au beam and Si substrate, the vertically and horizontally aligned Au lines came in contact and were electrically shorted, thereby forming a 2D mesh pattern. This allowed for the broadband THz modulation of 70% at 30 V for a bandwidth of over 1.5 THz, as shown in figure 4(i). A variant of fixed-fixed beam, which is a membrane suspended by anchors was reported for spectrally tunable THz absorbers [145]. In all of the aforementioned approaches, the deformation range is quite limited due to the achievable thickness of sacrificial layers and the direction of actuation is always towards the substrate due to the attractive nature of electrostatic actuation. For certain applications, such as chirality switching, spiral microstructures need to be reconfigured between planar 2D spirals to 3D helices. To realize such large deformation, a novel scheme was reported by assembling a Si substrate in front of released spiral cantilevers with desired gap between them [146]. Upon applying voltage between the assembled Si substrate and suspended spiral cantilevers, the as-fabricated planar spirals were reconfigured into a 3D helices with a strong switching of chiral response, as shown in figures 4(k)–(m) respectively. All of the aforementioned works clearly highlight the key advantages of electrostatic actuation, including simpler integration, versatile design, low power operation and multichannel control to enable wide range of high-performance THz MEMS metadevices.

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Comb drive actuator is the most popular choice of electrostatic actuators that enable large in-plane deformation [147]. Comb drive actuator consists of a pair of interdigited overlapping fingers, in which one set of fingers is free to move, while the other set remains fixed. When voltage is applied between the two sets of fingers, the electrostatic field between the fingers drives the movable set along the in-plane direction. Electrostatic comb drive actuators can provide continuous and large displacement and hence have been extensively used for in-plane reconfigurable THz MEMS metadevices [148]. In an initial report, a pair of SRRs formed a unit cell of which one SRR was fabricated on a movable Si frame driven by electrostatic comb drive actuators, while the other SRR was fabricated on the fixed Si island, as shown in figures 5(a) and (b) [149]. The SRR housed on the frame could then be moved either towards or away from the fixed SRR, thereby forming either an open-ring state, closed-ring state or back-touch state configuration of the unit cell, as shown in figures 5(c)–(e), respectively. The bidirectional reconfiguration was achieved by using two comb drive actuators fixed on either sides of the movable Si frame, as shown in figure 5(a). The as-fabricated metadevice unit cell consists of two SRRs with two capacitive gaps on either sides, as shown in figure 5(b). When THz wave with TE polarization was incident at 45°, Fano-type magnetic resonance was observed at ∼2.12 THz, as shown in figure 5(f). By increasing the capacitive gap from 2 µm to 6 µm between the SRRs, the resonance frequency was blue-shifted from 2.12 to 2.28 THz, respectively, as shown in figure 5(f). When the in-plane gap between the SRRs was completely closed i.e. close-ring state, no resonance was observed as shown in figure 5(g). Similarly, when back-touch configuration was formed by actuating the SRR in opposite direction, no resonance was observed as well, as shown in figure 5(h). Hence, the in-plane reconfiguration allowed for switching of magnetic resonance as well as spectral tuning of magnetic resonance frequency of the THz MEMS metadevice. The versatility of this platform allowed for the active control of various THz functionalities. Anisotropy switching was reported in a maltase cross metamaterial driven by comb-drive actuator [150]. This was realized by selectively breaking the symmetry of the cross structure along one-direction, while preserving it along the orthogonal direction in all reconfiguration states as shown in figure 5(i). In another work, the THz response of the metadevice was actively switched between polarization-dependent state and polarization-independent state by altering the lattice constant of the unit cells through in-plane reconfiguration as shown in figure 5(j) [151]. The as-fabricated metadevices had varying pitch along x-direction and y-direction and showed polarization-dependent THz response. Alternate lines of the metamaterial were fabricated on a movable frame connected to a set of comb drive actuators. By changing the relative position of the unit cells in the neighbouring lines, the lattice period was shifted to form interpixelated supercell with four fold-rotational symmetry and hence led to polarization-independent THz response. The comb drive integrated THz MEMS metadevice was also reported for polarization conversion and rotation by forming staircase structures with UP, 'T' and DOWN states [152]. The metamaterial consists of horizontal metallic slabs on fixed Si islands and the vertical metallic slabs were fabricated on the movable Si frame. By selectively displacing the vertical slabs, the staircase structure could be reconfigured between the UP, 'T' and DOWN states as shown in figure 5(k). The polarization rotation angle was reconfigured between −12.8°, 0° and +13.1° at 1.78 THz corresponding to UP, 'T' and DOWN states of the staircase unit cell. Comb drive actuators were also reported for dynamic control of broadside coupling in dual layer SRRs [153]. The metadevice consists of two arrays of SRRs stacked in a broadside-coupled manner [154]. The unit cell consists of a pair of 180° rotated SRRs separated by an out-of-plane gap as shown in figure 5(l). Due to broadside coupling, mode splitting was observed. When the relative overlap between the two SRRs was varied, a large blue-shift in symmetric mode and large red-shift in anti-symmetric mode was observed. Furthermore, strong amplitude modulation and phase shift were also achieved at specific THz frequency. Comb drive based THz MEMS metadevices have also been reported for reconfiguration of in-plane coupling in asymmetry SRRs [148]. In a more recent work, comb drive integrated substrateless metadevice was also reported for active tuning of EIT analogue [155]. Hence, comb drive actuators provide a powerful platform for in-plane reshaping of metamaterial unit cell geometry as well as lattice reconfiguration to achieve versatile THz functionalities.

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The primary property of any flexible substrate is the ability to bend upon applying mechanical force. Some of the popular flexible substrates used for THz metamaterials include polyimide [17, 31, 69, 73, 162–166], polydimethylsiloxane (PDMS) [167–170], polyethylene terephthalate (PET) [50, 171], parylene [172], polyethylene naphthalate (PEN) [173, 174], polypropylene [175], silicone [176] and silk [177–180]. In an initial work, metallic SRRs fabricated on PEN substrates were rolled into 3D tubes with varying diameter, as shown in figure 8(a) [173]. The resonance frequency of the planar SRR was measured at 0.75 THz, as shown in figure 8(b). When the metadevice was rolled into 3D tubes with decreasing diameter, the resonance frequency gradually blue shifted to 1.13 THz for the diameter of 4 mm, as shown in figure 8(b). In addition, the hollow core of the metamaterial was replaced with dielectric materials and the change in THz resonance frequency was used to estimate the refractive index of the core material. The high sensitivity of the metamaterial allowed for the detection of minuscule refractive index change of ∼0.0075. In another work, symmetric Au ESRR metamaterial was sandwiched between two PDMS layers and upon bending, the resonance frequency of the metadevice gradually blue shifted with increasing curvature [181]. Hence, bending of flexible substrates provides an straightforward means of achieving reconfigurability without the need for active materials, complex processing or electrical interconnections. They also enable extremely low-loss and low-cost metadevices.

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A subset of flexible substrates such as PDMS are elastic in nature and hence expand upon applying in-plane strain. This property of elastic substrates has also been used for realizing in-plane reconfigurable THz metadevices [168, 169, 182, 183]. Hexagon-shaped metallic resonators in honeycomb lattice were fabricated on a thin polyimide substrate and were then transferred to a prestrained PDMS substrate. Upon releasing the strain, the PDMS layer with metallic resonators wrinkled up, as shown in figure 8(c) [182]. Mechanical strain of 52.5% was applied to flatten the wrinkled layout as shown in figure 8(d). The relative change in THz transmission at 1.5 THz was changed continuously and a maximum change of 80% was observed between the as-fabricated wrinkled and strain-applied flat layout of the metadevice, as shown in figure 8(e). Similarly, closely spaced 'I'-shaped resonators, fabricated on PDMS substrate was also reported for mechanically reconfigurable THz response [168]. Upon applying mechanical strain, the intercell capacitance along the length of 'I' direction decreased, thereby causing the resonance frequency of the metadevice to red shift. The spectral tunability of the metadevice was improved by adopting a corrugated 'I'-shaped resonator, which provided larger intercell capacitance compared to the plain 'I'-shaped resonators. The 'I'-shaped metadevices were polarization sensitive. To enable polarization insensitive response, a cross resonator with corrugated capacitive gaps was reported [169, 184]. Hence, mechanically stretchable substrates provide a simpler route to achieve in-plane reconfigurable metadevices, that are essential for applications demanding non-conformal coatings, such as wearable devices and stealth technologies.

SMAs are phase transition materials that can be switched between two stable phases in response to applied temperature. One such SMA is nitinol (NiTi), a metal alloy composed of approximately equal atomic percentages of nickel (Ni) and titanium (Ti). NiTi shows phase transition between the martensite phase below the transition temperature and the austenite phase above the transition temperature, respectively. The phase transition temperature of NiTi is close to room temperature. NiTi also has very high DC conductivity of ∼1.2 × 10⁶S m⁻¹ in both the states. Hence, subwavelength-sized holes perforated in NiTi was reported as a phase transition powered THz metadevice [185]. The SMA was trained to achieve both 1D and 2D sinusoidal corrugation upon phase transition. When the applied temperature was lower than the martensite phase transition temperature, the NiTi hole array formed a 1D sinusoidal pattern as shown in figure 8(g). This resulted in polarization-dependent THz transmission response for E|| and Eperp incidence of THz waves with transmission amplitude of ∼0.45 and 0.25, respectively, as shown in figure 8(i). When the temperature was raised above the austenite phase transition, the NiTi metadevice became flat as shown in figure 8(f). The corresponding THz response was polarization-independent for both E|| and Eperp THz incidence with maximum transmission of ∼0.55 at 0.45 THz as shown in figure 8(h). Similarly, an aperture was formed in the NiTi film and was trained to have 2D sinusoidal corrugation around the aperture in the martensite phase. The THz transmission was increased to almost unity at 0.1 THz, while for the bare aperture on a flat NiTi film, the transmission was only 0.45 THz. Hence, phase transition in SMA was shown to be an efficient platform to achieve THz MEMS metadevices with memory effects and trainable properties.